We have evidence suggesting that neonatal activation of the pituitary- testicular axis is a critical event in the process of sexual and behavioral maturation in male primates. Monkeys treated as neonates with a gonadotropin-releasing hormone (GnRH)-agonist to suppress testicular function exhibited differences in social behavior during the second year of life, a delay or retardation of testicular development during the peripubertal period and reduced male mountings during the non-breeding season of their third year of life. The proposed study is an effort to expand our understanding of the importance of neonatal testosterone as an organizational influence on sexual and behavioral processes in male primates. There are two components to this proposal: an adult component involving 5-year-old male monkeys treated as neonates with a GnRH-agonist and an infant component involving the development of an additional cohort of neonatal hypogonadotropic-hypogonadal animals using a potent GnRH- antagonist. With the adult component, we will determine whether the deleterious effects of suppressing neonatal pituitary-testicular function on sexual and aggressive behavior under conditions of high and low intermale competition are altered by neonatal treatment with a GnRH- analogue. Efforts will be made to define the site(s) (hypothalamus, pituitary or testis) of the physiological defect and determine whether the FSH-inhibin negative feedback loop is functionally normal in GnRH-agonist- treated monkeys. We will also perform a detailed evaluation of the fertility of these animals. In the infant component, we will attempt to confirm the effect of blocking neonatal activation of the pituitary- testicular on sexual maturation using a GnRH-antagonist. The importance of neonatal testosterone in this process will be assessed by treating one group of infants simultaneously with the GnRH-antagonist and testosterone. The effect of treatment on the developmental pattern of pituitary and gonadal hormone secretion, on growth and maturation of the skeletal system and on sexual maturation will be determined. The development of male typical behavior and adult sexual behavior will be compared between animals treated with the GnRH-antagonist alone or in combination with testosterone therapy. The proposed study will increase our understanding of developmental mechanisms that govern sexual and behavioral development in male primates and the role that neonatal testosterone plays in this process.